1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission which is useful for operating during engine braking.
2. Description of the Prior Art
A line pressure P1 is generally adjusted so as to increase within a given range in association with an increase in the degree of opening of a throttle valve in a carburetor. At a high degree of the opening of throttle valve in the carburetor, a downshift point from the second speed drive position to the first speed drive position is shifted to a high speed side, thus causing the over run of an engine when it is desired to operate engine braking. In order to prevent the over run of the engine and to effect the downshift at a constant speed of a vehicle irrespective of a degree of an opening of the throttle valve, it has been customary to provide valves or the like in parallel to a 1-2 speed shift valve. This, however, results in an increased width of the valve body. Furthermore, where a spool is oscillated when the hydraulic pressure and the spring force are in equilibrium so as to adjust the pressure, foreign substances may get between the inner peripheral wall of the valve body and a land of the spool, thus leading to a seizure of such members.